This invention relates to a processor for a radar system, particularly for a synthetic aperture radar (SAR) system.
Synthetic aperture radar is a known technique which is suitable for producing a high resolution radar image of a terrain under a moving radar platform, e.g. an aircraft or satellite. A typical synthetic aperture radar system will now be described with reference to FIGS. 1 and 2 of the accompanying drawings.
Referring to FIG. 1, the system comprises a radar carried on a moving platform 1, which looks approximately sideways to the path of motion 3 of the platform 1. As the platform moves along the path 3, the radar periodically transmits radar pulses and collects the echoes. A pulse illuminates an area 5 of the terrain beneath the platform, which is commonly referred to as the radar footprint. The collected radar echoes are processed by the radar system to produce the required radar image.
As the platform moves along the path 3, the footprint moves along a strip 7 of the terrain referred to as the illuminated swath, the direction along the length of the swath being referred to as the azimuth direction and the direction across the width of the swath being referred to as the range direction. Radar echoes are received in the time between successive radar pulses, as shown in FIG. 2, in a data reception window 11, and the echo signal is sampled (range line).
The resolution of the image obtained in the range direction is primarily determined by the bandwidth of the transmitted pulse, a high resolution being obtained with a high bandwidth pulse using known range processing algorithms, which correlate the range lines with a replica of the transmitted pulse signal.
High resolution can be obtained in the azimuth direction by choosing the pulse repetition interval such that successive footprints overlap. The range line signals obtained from each set of overlapping footprints are processed simultaneously (being correlated or combined in another suitable relation with data relating to the Doppler shift of the echo signals) and in this way, an azimuth resolution comparable to that obtainable with an antenna of much larger size and aperture is obtained.
A problem with such azimuth processing is the so-called range migration effect under which the distance between the platform and a point on the terrain illuminated by the beam changes from pulse to pulse. Consequently any such point is represented by range line samples at different positions on successive range lines, the positions defining a range migration path, which may be curved, or substantially linear. Since the correlation with the Doppler coefficients has to be carried out on data samples on the range migration paths, the range migration effect must be taken into account to perform the processing.
U.S. Pat. No. 4132989 is a proposal for range migration correction in azimuth processing, which entails the use of a number of shift registers, one for each sample range line, connected in cascade. Range migration correction is performed by applying a variable delay to the output of each shift register. The delayed outputs (corresponding to the same point on the ground) are then correlated with the Doppler reference coefficients.
To simplify the processing, it has been proposed in situations where the range migration curvature is not great, to approximate it to a straight line. For example, by a co-ordinate transformation, additional samples can be interpolated for each range line such that the points lie on slant or skew axes parallel to the straight line approximation of the range migration curve, rather than on axes parallel to the azimuthal direction. The points lying on the skew axes can then be correlated with the Doppler reference coefficients.
However, a disadvantage with this is that the original sample data lying in the included angle between the skew axis and the azimuthal axis will not be processed.
The applicants have realised that the advantages of straight line approximation and the maximum utilisation of data can be both achieved if data line samples commence from a skew axis of saw-tooth form in the region of the included angle.
The invention provides an azimuth processor for a synthetic aperture radar system, which comprises a sequence of data storage locations, and means for entering successive range line samples derived from successive radar echo signals into the sequence of storage locations, at a variable starting location along the sequence.
The starting location at which successive range line samples are entered can be changed so that the discontinuities in the saw-tooth form of data will be transparent to the processor, enabling the processing advantages of this form of data to be achieved.
The range line samples entered into the sequence of storage locations may be samples of radar return samples after range processing e.g. by correlation with a replica of the transmitted pulse.
Advantageously, columns of data are derived from the data successively fed into the input sequence of data storage locations, Thus, the data may be transferred from the input sequence to a two-dimensional array of storage locations, and samples may be read from the array for azimuth processing. The array may be divided into modules. The relative lengths of rows and columns in the array may be variable, in order to trade the length of the swath being measured against the azimuth resolution provided within that swath. The two dimensional array may be formed by a data memory which could also include the input row and/or an output row.
The processor may include a linear range migration interpolator for performing a co-ordinate transformation to interpolate samples on a co-ordinate axis system in which one axis is skew compared to the azimuth direction. Such a co-ordinate transformation enables azimuth processing to be carried out along columns in the two-dimensional array where the range migration is linear (and indeed even the co-ordinate transformation would be unnecessary if there was no range migration). However, if the range migration path also has a non-linear component azimuth processing can take place along range migration curves by use of a further range migration processor of the form described and claimed in our co-pending application Ser. No. 07,233,251 filed Aug. 17th, 1988, now U.S. Pat. No. 4,879,559. Alternatively, the curvature can be approximated to a series of linear sections, and azimuth processing can be carried out on samples interpolated by the linear range migration interpolator on skew axes parallel to each linear section. The results can be combined to yield azimuth processed samples.
The sequence of data storage locations may be arranged in the form of a closed loop, in that the entire sequence will be filled and in the same order, irrespective of the starting position. Thus, in the case of a row, after the row has been filled from the starting point to the end, the storage locations at the start of the row are filled in succession. Advantageously, the storage locations are configured as a plurality of groups, and successive range line samples are fed in use to a location in each group in turn. This gives processing advantages, because when columns of data derived from the data successively fed into the groups are azimuth processed, columns of data derived from the different groups will require the same reference coefficients at any given time for azimuth processing. This enables the necessary reference coefficients to be stored in a single memory. An output row of storage locations for processed data may be provided, which can be read from a variable starting point along the row which may move in step with that for the input row: this provides output data in the same structure as the input data. The input row could be part of an input buffer, which could consist only of that row, or could consist of more than one row. Equally, the output row of storage locations could be part of an output buffer, which could consist only of that row, could consist of more than one row.
Instead of sequence of storage locations being filled by a moving pointer, the sequence of storage locations may be formed by a shift register.
The invention also provides an azimuth processor for a synthetic aperture radar system, comprising means for performing a co-ordinate transformation on range line samples derived from successive radar echo signals, so that samples are interpolated on a skew axis parallel to a straight line approximation of the range migration curve, and means for azimuth processing such data wherein the skew axis is of saw-tooth form.
The invention also provides a processor for two-dimensional separable convolution/correlation of an array of samples, comprising a correlator or convolver to correlate or convolve each line of input samples from the array, a sequence of data storage locations for storing the correlated or convolved lines of samples, a two-dimensional array of storage locations arranged to receive successive rows of samples from the data storage locations, and correlators or convolvers arranged to correlate or convolve the samples in the columns in the array.